The present invention relates to methods and compositions for inhibiting the polymerization of diene monomers, and more particularly relates, in one embodiment, to methods and compositions for inhibiting the polymerization of butadiene monomers by decomposing hydrogen peroxide.
In the production of olefins, such as a diene, the so-called popcorn polymer having a porous, three-dimensional structure occurs frequently and undesirably in process apparatus due to the unintentional polymerization of the olefin. Popcorn polymerization occurs in refining and distillation of the monomer, or during recovery of the monomer after termination of intentional polymerization such as during the production of poly-butadiene and synthetic rubber, in particular styrene-butadiene rubber. Popcorn polymerization occurs both in the gaseous phase and the liquid phase. It is more likely to occur when the concentration of the olefin monomer is high and the temperature is high. Trace amounts of peroxides used to intentionally generate free radicals to initiate and perpetuate polymerization may remain in these systems to produce free radicals that cause polymerization at undesirable locations.
Numerous olefin monomers such as styrene, a-methyl styrene, acrylic acid and esters thereof, vinyl acetate, acrylonitrile, acrylamide, methacrylamide, etc. and such dienes (diolefins) as 1,3-butadiene, isoprene, and chloroprene, upon reaching refining devices during production and recovery, are exposed to certain conditions such as high temperature, high monomer concentration, coexistence of vapor and liquid phase, humidity, trace oxygen and iron rust which are highly conducive to the occurrence of popcorn polymerization. Fouling of equipment can even occur when unsaturated compounds in petroleum or its derivatives undesirably polymerize.
In the manufacturing of synthetic rubber and plastics for use by the military and others, the primary choice of feedstock is butadiene. Butadiene (BD) is a colorless gas at room temperature. Most of the supply of butadiene comes from olefin plants because butadiene is co-produced when other olefins are manufactured. Butadiene can be produced by catalytically dehydrogenating butane or butylene.
The dehydrogenating of butane or butylene to butadiene may be accomplished by passing the feed gas over a catalyst bed at 1200xc2x0 F. (649xc2x0 C.) and at reduced pressure. The effluent gas then passes through an extractive distillation process.
Although butadiene can be manufactured by catalytic dehydrogenation, most of the butadiene produced domestically is obtained by extractive distillation. Butadiene can be recovered and purified from C4 streams by using a solvent that reduces the boiling point of the butadiene. The most popular solvents used to facilitate this extraction are N-methylpyrrolidone (NMP) and dimethylformamide (DMF).
The crude butadiene is further purified by distillation through a series of towers that separate residual solvent, C4 compounds, and other contaminants. A further component of the BD manufacturing process is the Recovery Solvent Section. From the stripper column, a slip stream of lean solvent is regenerated to remove heavies and contaminants. This reduces the presence of polymer loading and decomposition products.
During the purification steps of BD production, there are some undesirable BD polymerization reactions. Of these, the most unwanted is the formation of popcorn polymer. The effect of popcorn polymer on BD processing equipment is so severe that this material has actually been found to bend heat exchanger tubes.
Poly-butadiene can be produced commercially by way of an aqueous homopolymerization reaction. In order to actuate this polymerization process, hydrogen peroxide is used in excess of 7400 ppm to initiate butadiene to react to form poly-butadiene (Poly-BD). However, the presence of peroxides and trace levels of Poly-BD in the water from the decanter drum used in the process results in undesirable polymerization at the acetone column. Presently, Poly-BD manufacturers use sodium sulfite to control peroxide levels in the feed to the acetone drum, which feeds the acetone column, but this program is not very efficient in reducing peroxide levels. Also, other chemical suppliers have attempted to treat the polymerization problems with commercial antioxidants, but have experienced very little success. This failure can be linked directly to the economic limitations on inhibitor usage to control such relatively large concentrations of peroxides. This results in relatively small, but still significant amounts of hydrogen peroxide and other peroxides that are not destroyed, but which in turn results in undesired polymerization in the acetone column. Currently commercial Poly-BD producers can spend large sums annually on the use of sodium sulfite that is capable of removing only about 50% of the residual peroxides.
It would be desirable if a composition and method could be devised to inhibit the undesirable and unintentional polymerization of unsaturated (vinyl) monomers, particularly butadiene, but without having to require relatively large, and therefore costly, amounts of inhibitors.
Accordingly, it is an object of the present invention to provide a method and composition to effectively inhibit the polymerization of vinyl compounds, such as butadiene.
It is another object of the present invention to provide a method and composition to effectively inhibit the polymerization of olefins that is less expensive than using conventional antioxidant inhibitors because relatively less amount of the inhibitor is required to achieve acceptable polymerization inhibition.
Still another object of the invention is to permit use of a composition to effectively inhibit the polymerization of butadiene that uses readily available compounds.
In carrying out these and other objects of the invention, there is provided, in one form, a method for inhibiting the polymerization of unsaturated monomers that are polymerized by free radicals generated by peroxides, the method comprising adding to the unsaturated monomers an effective polymerization-inhibiting amount of an inhibitor selected from the group consisting of diamines, alkanolamines, and mixtures thereof.